After freeing Queen Celestia's sister from the pall of NightMare Moon, Twilight Sparkle has settled down in Ponyville with her new circle of friends. But no sooner does she settle down than a foe from the reign of NightMare Moon returns: Trixie Lulamoon! Bearing a mysterious trinket of great magical power, Trixie leads Twilight to test her assessment of friendship being the greatest magic. What is the Alicorn Amulet? How does it possess such absolute power? Where did Trixie get it from? Who gave it to her, and why? And what was the Sonic Rainboom, and how does it tie Twilight Sparkle to the friends she has just made...?

Can be found on TV Tropes.

The Elements of Friendship, Book I: Harmony
The Elements of Friendship, Book IS [interlude one-point-five]: Bonds
The Elements of Friendship, Book II: Chaoskampf
The Elements of Friendship, Book IIS [interlude two-point-five]
The Elements of Friendship, Book III
The Elements of Friendship, Book IIIS [interlude three-point-five]
The Elements of Friendship, Book IIII